Conventionally, while connecting metallic materials by welding, metallic pieces as so-called spatters scatter around and cause phenomena as protrudent matters adhered to surfaces of metallic materials to be connected. Since the protrudent adhered matters worsen a normal surface of the metallic material and possibly cause deterioration of a product quality, it has been demanded to establish a method of generating no protrudent adhered matters when connecting metallic materials, or a method of exactly detecting presence or absence of the protrudent adhered matters in products.
A problem to be solved by the invention is to provide a method of detecting the protrudent matters adhered to the surfaces of metallic materials when connecting them and a method of making spark plugs using the same, enabling to detect at high precision occurrence of the protrudent adhered matters caused by connecting the metallic materials, and in turn offer products of high quality, and effectively accomplish increase of yield.